The invention relates generally to a method of rapid prototyping and manufacturing and, more particularly, to laser sintering.
Rapid prototyping and manufacturing (RP&M) is the name given to a field of technologies that can be used to form three-dimensional objects rapidly and automatically from computer data representing the objects. In general, rapid prototyping and manufacturing techniques build three-dimensional objects, layer-by-layer, from a working medium utilizing sliced data sets representing cross-sections of the object to be formed. Typically an object representation is initially provided by a Computer Aided Design (CAD) system. RP&M techniques are sometimes referred to as solid imaging and include stereolithography, ink jet printing as applied to solid imaging. and laser sintering, to which the invention is directed.
Laser sintering apparatus dispenses a thin layer of fusible powder, often a fusible polymer powder or polymer coated metal, over a bed of the powder and then applies thermal energy to melt those portions of the powder layer corresponding to a cross-section of the article being built in that powder layer. Lasers typically supply the thermal energy through modulation and precise directional control to a targeted area of the powder layer. Conventional selective laser sintering systems, such as the Vanguard system available from 3D Systems, Inc., use carbon dioxide lasers and position the laser beam by way of galvanometer-driven mirrors that deflect the laser beam. The apparatus then dispenses an additional layer of powder onto the previously fused layer and repeats the process of melting and selective fusing of the powder in this next layer, with fused portions of later layers fusing to fused portions of previous layers as appropriate for the article, until the article is complete. These articles are sometimes referred to as “built parts.”
A computer operates the control system for the laser, programmed with information indicative of the desired boundaries of a plurality of cross sections of the part to be produced. The laser may be scanned across the powder in raster fashion or vector fashion. In vector fashion, the laser beam traces the outline and interior of each cross-sectional region of the desired part. In a raster scan, a modulated laser beam scans a repetitive pattern across the powder. In some applications, cross-sections of articles are formed in a powder layer by fusing powder along the outline of the cross-section in vector fashion, either before or after a raster scan that “fills” the area within the vector-drawn outline.
Detailed descriptions of laser sintering technology may be found in U.S. Pat. Nos. 4,863,538; 5,132,143; and 4,944,817, all assigned to Board of Regents, The University of Texas System, and in U.S. Pat. No. 4,247,508 to Housholder.
Laser sintering technology enables the direct manufacture of three-dimensional articles of high resolution and dimensional accuracy from a variety of fusible materials, including polystyrene, some nylons, other plastics, and composite materials, including polymer coated metals and ceramics. Laser sintering may be used for the direct fabrication of molds from a CAD database representation of the object to be molded. Computer operations “invert” the CAD database representation of the object to be formed to directly form the negative molds from the powder.
Laser sintering depends upon thermal control of the process in the part cake to obtain good three-dimensional parts. The sources of thermal energy are the laser, cylinder heaters that preheat the powder in powder feed cylinders that supply a powder layer to the apparatus, radiant heaters to heat the powder prior to deposit on the laser target area, the radiant heater for the laser target area, and the laser. The laser is typically a CO2 laser that scans the fresh powder layer to fuse the powder particles in the desired areas.
The increasing number of applications for laser sintered products has resulted in a demand for built parts having improved physical properties. Present commercial systems effectively deliver powder and thermal energy in a precise and efficient way. Nevertheless, laser sintered parts are sometimes dimensionally distorted and may not have the strength of, for example, injection molded plastic parts.
The sintering process may leave void spaces between the individual particles that reduce the strength of the built part. Increasing the thermal energy supplied to the fusible powders can result in dimensionally distorted parts. Heated particles at the boundaries of the target area may melt and adhere to particles immediately outside the targeted area. The interior of individual powder particles may become melted causing excess material to flow into void spaces that exist between the surrounding particles. One or more layers may experience an overall increase in dimensions from the nominal values calculated by the CAD program. The undesirable increase is commonly referred to as “growth” and reflects that the mean value of the dimensions obtained varies an unacceptable degree from the calculated nominal value. Such growth may make a sintered part unusable for its intended purpose.
Thus, there exists a need for a method of using laser sintering to produce parts that are accurate and have high strength.